1. Field of the Invention
The present invention relates generally to the art of press bending glass sheets and more particularly to the art of press bending glass sheets having a printed portion.
Automobile backlights are typically provided with a painted border portion to enable them to be mounted flush with a curved frame of an automobile. In such so called "flush mounting," the painted border portion conceals the frame without requiring metal strips or the like to conceal attachment elements that secure the backlight to the frame.
In the glass industry, tempered backlights to be mounted flush with the automobile frame so that the shape and outline of the curved backlight merges with the automobile frame, are fabricated by first applying the paint to the desired portion or portions of the glass sheet, gripping the painted glass sheet by tongs, conveying the tong-gripped glass sheets through a tunnel-like furnace where each glass sheet is heated to a temperature sufficient for tempering, engaging the tong-gripped glass sheets between a pair of press bending molds having complementary shaping surfaces that impress onto the heat-softened glass sheet a desired shape and then separating the molds and passing the press bent glass sheet through a cooling area where blasts of tempering medium, such as cold air, are applied sufficiently rapidly over the opposite major glass sheet surfaces to impart at least a partial temper in the glass.
When glass sheets are tempered, they are heated to above the stain point of the glass, and even as high as a temperature approaching the glass softening point. After the glass attains a temperature sufficient for tempering, it is chilled rapidly. Stresses are established temporarily in the glass during this rapid cooling. These stresses may become so severe as to cause the glass sheet to fracture during the tempering operation. Glass fracturing during tempering has been reduced to some extent by providing the press bending molds with covers of flexible fiber glass that insulate the relatively hot glass sheets from the relatively cold press bending molds. Such insulation reduces the tendency of the press bent glass sheet to develop steep thermal gradients due to non-uniform contact with the press bending molds. These steep thermal gradients have been associated with glass sheet breakage during the fabrication of bent tempered glass sheets by press bending followed by rapid chilling.
Unfortunately, when press bending molds provided with flexible fiber glass covers are used to shape glass sheets having a painted portion, the paint tends to transfer from the glass sheet to the cover. Paint on the cover is likely to transfer to a subsequent glass sheet in a mass production press bending operation. Sometimes, the paint is deposited in an area where the bent, tempered backlight is desired to be free of any defect such as a paint mark.
Consequently, it would be desirable to be able to avoid the transfer of paint from the painted portion of a glass sheet to be press bent and tempered to the cover for the press bending mold. Since other bent tempered glass sheets are provided with painted portions other than in the border region, it would be equally desirable to avoid the application of paint marks to glass sheets to be press bent and tempered regardless of the location of the desired paint application.
In addition to the above-described defects relating to the marking of glass sheets, another problem encountered in the press bending operation is the difficulty of separating the bent glass from the press bending molds when the latter separate after having sandwiched the glass sheet therebetween to impress the desired shape into the glass sheet by pressurized engagement. The press bent glass sheet tends to stick to one of the press bending molds when the molds separate. Usually, the press bent glass sheet remains against the protruding portion of the convex press bending mold when the press bending molds have complementary shaping surfaces of convex and concave configuration. It would be desirable to overcome this problem, because any delay in transferring the glass sheet from the press bending molds to the cooling area reduces the maximum temper obtainable in the press bent glass sheet.
2. Description of Patents of Interest
U.S. Pat. No. 2,560,599 to Ryan discloses press bending molds comprising fiber glass fabric impregnated with plastic or resinous filler.
U.S. Pat. No. 3,148,968 to Cypher and Valchar discloses press bending molds having covers of knit fiber glass cloth composed of texturized yarn.
U.S. Pat. No. 3,420,652 to Seymour discloses a cover of fiber glass cloth supporting an embedded wire fabric in an area that faces a decorative painted strip on a glass sheet to be bent by press bending.
U.S. Pat. No. 3,713,798 to Stilley, Wagner and Kely discloses treating a fiber glass cloth cover of a press bending mold with a dispersion containing a Werner complex salt to improve the durability of the cover.
In U.S. Pat. No. 3,492,109 to Robinson et al, boron nitride is disclosed for use in glass engaging rolls of a float glass tank.
In U.S. Pat. No. 3,783,013, Seeman discloses coating metal glassware molds with a dispersion of 12 to 53 percent by weight monoaluminum phosphate and 47 to 88 percent by weight boron nitride.
An asbestos roll with a boron nitride coating found to prolong roll life and to minimize deformation of the roll and adhesion to the glass surface is disclosed in U.S. Pat. No. 3,853,525 to Gorman. The roll is prepared by adhering asbestos discs together on a mandrel, spraying on a solution of boron nitride, air drying for one to ten hours at room temperature to 200.degree. F. (93.degree. C.), then baking at 200.degree. to 1500.degree. F. (93.degree. to 815.degree. C.) for two to six hours before installing the roll in a glass producing apparatus.